The present invention relates generally to an apparatus and method for removing moisture from composite materials and, more particularly, to an apparatus and method for removing moisture from the interior of composite helicopter rotor blades. Natural environmental changes allow composite rotor blade structures to absorb and hold moisture. The present apparatus and method will quickly and economically reverse the natural environmental cycle and remove the moisture trapped within the blades. The present apparatus and method can also be utilized in the field.
First generation helicopter blades were constructed of wood and fabric. Unfortunately, over time, wooden blades deteriorated due to dry rot, moisture absorption which caused swelling of the blades, and such blades generally suffered from all of the typical environmental effects common to wood products. Second generation rotor blades were manufactured from aluminum. These blades also suffered from environmental effects, namely, corrosion as well as the fact that they were intolerant to damage and even small anomalies located in a critical structural area could quickly fatigue the blade leading to potentially catastrophic results.
Composite rotor blades were developed during the 1970s to overcome problems inherent in the use of metal blades. Today, military helicopters utilize nearly exclusively composite rotor blades because of their tolerance to combat damage and resistance to in-flight structural failure. Composite blades are structurally redundant and can take tremendous structural damage without catastrophic failure. These blades are also easily repaired in the field. Typical composite blades utilize a Nomex honeycomb core with bonded glass skins forming the aft fairing portion of the blade. The Nomex core is oriented with the open lines of the cells bonded to the top and bottom glass skins. For example, the fiberglass rotor blades of a Boeing CH-47D helicopter are composite glass structures. The D shaped spar of such blades is a glass lay-up with a titanium erosion strip. The aft fairing is a Nomex honeycomb and glass skinned structure. These rotor blades are painted with a conductive xe2x80x9cstaticxe2x80x9d coating and finished with a lacquer. Improved coatings include the use of polyurethane in place of lacquer due to improved flexibility and moisture sealing capability.
Overtime, the glass skins associated with composite rotor blades are likewise subjected to the effects of the environment and to hard usage. With time, the ultra-violet effects of the sun, erosion, poor repairs and repeated high cyclical loading attack the epoxy glass matrix causing minute openings in the skins. In addition, installed blades on helicopters are normally tied down in an outdoor environment with the top skins of the blades under constant tension. In this situation, weakened areas associated with the top skin of the blades can open up during tie down allowing moisture to enter. Still further, a rotor blade sitting outside in the sun can easily reach 180xc2x0 F. When heated, the air inside the Nomex honeycomb core expands and leaks out through any fissures in the skin. Upon cooling, the pressure with in the honeycomb core will decrease, and the air will be pulled back into the blade, along with any moisture present on the surface of the blade, including rainwater, condensation or humid air. Water is therefore accumulated within the blades over long periods of time, usually in small quantities through each cycle. The accumulation is worst in moist tropical climates. Significant quantities of water can be accumulated and stored in the core cavities of the blade in this manner.
Over time, this moisture upsets the track and balance of the blades and becomes an operational problem. When blades with water become too heavy to be balanced, they are currently sent to an overhaul depot where x-ray equipment is used to locate the moisture. Locally cutting the blade skin, routing out the affected core section and replacing the damaged area with a core plug and skin patch is the usual repair. Unfortunately, x-ray methodology is expensive and time consuming, and requires a highly skilled technician. Accurate mapping of the x-ray film to the blade as well as managing adequate x-ray coverage is important. Continuous x-ray equipment similar to the equipment used in airport security stations mitigates some of these problems but the expense of such equipment is usually prohibitive and precludes all but the major depots from accomplishing the necessary water removal and repairs.
Finally, when x-ray methods do, in fact, reveal water accumulation over a high percentage of the blade area, drilling of individual cells and coring becomes cost prohibitive and the blade is normally scrapped. A replacement blade for a typical military helicopter typically costs in the neighborhood of between $85,000 and $105,000.
It is therefore an object of the present invention to provide an economical method and apparatus to locate the moisture present in a particular rotor blade, to remove the moisture present in the rotor blade, and thereafter to accomplish any further repairs as necessary.
Another object of the present invention is to provide a method and apparatus which remove moisture from rotor blades non-invasively, resulting in lighter, superior blades.
Another object of the present invention is to provide a method and apparatus for removing moisture from rotor blades which can be utilized in the field within an acceptable period of time.
Another object of the present invention is to provide a method and apparatus which will more easily identify the location of the fissures and damage in the skin of the blade in preparation for repair.
Still another object of the present invention is to provide a method and apparatus for removing moisture from rotor blades which can be operated by technicians with minimal training.
To accomplish the foregoing and other objets of the present invention there is provided the following method and apparatus.
An atmospherically sealed container or xe2x80x9cboxxe2x80x9d is provided. The present box or container is built large enough to completely house the damaged blade or blades. Blades are set on racks or other support fixtures that hold the blades in an inverted position relative to the blades"" actual position when installed on the helicopter. The present support mechanism supports the blades by their opposite ends only as to allow the blades to bow slightly in the middle. This puts the top skin on each blade, which is facing down in the present box, in tension just as it was when installed on the helicopter. In this position, gravity works to pull water to the inner surface of the top skin. This positioning also tends to open any breaks or fissures present in the top skin.
A dehumidifier is installed in one end of the box. The dehumidifier has its dry air discharge piped to the other end of the box. The dry air then blows lengthwise over the blades back to the air inlet of the dehumidifier. Any moisture collected in this air as it travels across the length of the blades due to moisture being released from the interior of the blades is captured at an air intake to the dehumidifier and thereafter discharged to the outside of the sealed box. New dehumidified air is then again discharged at the opposite end of the present box and another cycle is initiated.
In order to speed up the drying process, heating the box and/or decreasing the pressure within the box can be used to reduce the overall time necessary to remove sufficient amounts of moisture from the blades. In this regard, cyclical heating and cooling of the box may more nearly reverse the natural process that allowed water to enter the blades in the first place, the heated air within the Nomex core expanding and tending to expel moisture trapped near the downside of the blades. In similar fashion, cycling the pressure within the box between atmospheric pressure and a pressure below atmospheric pressure will cause a pumping action to take place which will also tend to expel moisture from the downside of the blades.
As moisture exits the blade, talcum from the honeycomb core is washed to the surface clearly indicating and identifying the location of fissures through which the moisture entered/exited the blade. This gives a clear indication of where repairs are required. Proper sealing of the water entry points and repainting of the newly prepared surface will result in a water free blade that will resist moisture entry for a reasonable period of operational time. The present method and apparatus will therefore allow so called xe2x80x9cwater bladesxe2x80x9d to be repaired in the field.